Voltage-tunable magnetron



Sept. 20, 1966 D. 1... PETERSON 3,274,432

VOLTAGE-TUNABLE MAGNETRON Filed March 27, 1962 IN VEN TOR. DALE L. PETERSON ATTORNEY United States Patent 3,274,432 VOLTAGE-TUNABLE MAGNETRON Dale L. Peterson, Santa Clara, Calif., assignor, by mcsne assignments, to Varian Associates, a corporation of California Filed Mar. 27, 1962, Ser. No. 182,836 11 Claims. (Cl. SIS-39.63)

This invention relates to magnetrons, and particularly to a voltage-tunable magnetron.

Voltage-tunable magnetrons are susceptible to frequency instability from a variety of different causes. It is therefore one of the important objects of this invention to provide an inherently frequency-stable voltagetunable magnetron.

One of the causes of frequency instability in a voltagetunable magnetron is the utilization of a directly heated cathode. It is therefore one of the objects of the present invention to utilize an indirectly heated cathode in a voltage-tunable magnetron and thereby minimize undesired frequency instability in the magnetron.

Another source of frequency instability in voltage-tunable magnetrons is the tendency in conventional voltage tunable magnetrons to conduct heat from the collector and/ or cold cathode of the tube back to the hot cathode, and thereby increase the number of electrons being emitted from the hot cathode, resulting in variations in the power output of the tube. It is therefore another ob ject of the invention to provide a collector-cold cathode construction which is thermally insulated from the hot cathode.

Frequency instability in a voltage-tunable magnetron utilizing a direct-1y heated cat'hode also arises from the tendency of the directly heated cathode to vibrate. It is therefore a still further object of the invention to provide an indirectly heated cathode capable of withstanding great vibration and shock stresses without undesirable vibratlon.

In voltage-tunable magnetrons utilizing a directly heated filament-type cathode, back-bombardment current resulting from back-bombardment of the cold cathode by electrons usually passes through the filament cathode, thereby uncontrollably heating the filament and giving rise to undesired power output variations. It is accordingly another of the objects of the invention to provide a voltage-tunable magnetron in which the passage of backbombardment current to the hot cathode is eliminated.

Another undesirable characteristic of voltage-tunable magnetrons of conventional design involves the inability to secure a flat power output response from the tube between the time the tube is first turned on and the time when it reaches a state of steady output. In conventional voltage-tunable magnetrons this flat response is not possible because back-bombardment of the cold cathode heats the cold cathode, and heat from the cold cathode is conducted to the hot cathode, thus uncontrollably increasing the emission of the hot cathode, with consequent variation in the power output. It is therefore one of the objects of the invention to provide a voltage-tunable magnetron construction which provides a substantially flat power output response between the time it is turned on and the time when it reaches a state of steady output.

A still further object of the invention is to provide a voltage-tunable magnetron utilizing a three conductor circuit for connecting the interdigital anode segments with an external coaxial transmission line.

In conventional voltage-tunable rnagnetrons utilizing a two-conductor circuit to connect the interdigital anode segments to a coaxial transmission line, it is possible to utilize only the single groundplane lead connected to the outer conductor of the transmission line to conduct heat away from the anode segments. It is therefore another 3,274,432 Patented Sept. 20, 1966 object of this invention to provide a voltage-tunable magnetron in which a double ground plane connection is made between the anode segments and the transmission line so as to provide twice the thermal conductive path for extracting heat from the anode structure.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be apparent from the following description and the drawings. It is to be understood, however, that the invention is not limited to the embodiment illustrated and described, but may be incorporated in various forms within the scope of the appended claims.

Broadly considered, the invention comprises a voltagetunable magnetron fabricated as two separate subassemblies which after separate fabrication are hermetically united to form a composite tube structure. One of the subassemblies includes the collector electrode and a multiplicity of axially aligned tubular ceramic envelope sections between selected ones of which are interposed appropriate electrically conductive leads terminating within the envelope thermally and electrically connected to selected segments of an interdigital anode and to the control electrode. The collector includes an integral extension extending coaxia lly through the cylindrical array of interdigital anode segments to provide a cold cathode for the tube. The other subsassembly includes a metallic end Wall and a plurality of tubular ceramic envelope sections in substantial alignment with the tubular ceramic envelope sections of the first subassembly. Supported on the metallic end Wall is a centrally disposed thermally nonconductive metallic sleeves extending into the hollow interior of the cold cathode in coaxial radially spaced relation thereto, and into sliding engagement about an electrically and substantially thermally nonconductive guide and support post fixed on the collector. Mounted on the sleeve intermediate its ends is an indirectly heated cathode, heated by a suitable heater element supported in part on the sleeve. The separate subassemblies are proportioned so that when the subassemblies are united, the control electrode is operatively positioned with respect to the cathode, and the cold cathode is thermally and electrically insulated from the hot cathode to prevent the conduction of heat and back-bombardment. current from one to the other.

Referring to the drawings:

FIGURE 1 is a vertical half-sectional view taken in the plane indicated by the line 1-1 in FIGURE 2.

FIGURE 2 is a horizontal cross-sectional view taken in the plane indicated by the line 22 in FIGURE 1.

FIGURES 1 and 2 are drawn approximately five times actual size.

In more specific detail, the voltage-tunable magnetron embodying the invention comprises an evacuated envelope designated generally by the numeral 2, having a collector electrode 3 hermetically closing one end of the envelope and a closure plate 4 hermetically closing the opposite end of the envelope. The collector is preferably fabricated from oxygen-free, high-conductivity copper to provide a flat externally radiating surface 5 coextensive with the diameter of the envelope for more efficient heat dissipation, and is also provided With a centrally disposed integral extension 6 tubular in crosssection to provide a hollow chamber 7 therewithin closed adjacent the collector and having a free end 8 opening into the envelope at its end remote from the collector. The extension 6 functions as a cold cathode, and channels heat caused by back-bombardment to the large radiating surfaces of the collector. Centrally disposed within the chamber 7 and rigidly fixed to the collector as by brazing: is an electrically and substantially thermally nonconductive guide and support post preferably fabricated from aluminum oxide. The post extends as shown toward the open end of the chamber 7 and terminates intermediate the ends thereof.

The anode of the voltage-tunable magnetron is preferably of the interdigital type having a plurality of metallic anode segments 10 and 11 extending axially within the envelope and arranged in a circular radially spaced array concentrically about the cold cathode. In the embodiment illustrated, the magnetron preferably utilizes six such anode segments circumferentially equally spaced about the cold cathode to provide interaction gaps 12 therebetween and an annular interaction space 13 between the cold cathode and interdigital array of anode segments. Segments 10 are preferably electrically and thermally connected to each other within the envelope by a pair of parallel axially spaced radially extending annular flanges 14, which terminate outside the envelope in terminal leads 16. These terminal leads constitute a highly thermally conductive double ground plane adapted to be electrically connected outside the envelope to the outer ground potential conductor of an associated coaxial transmission line. Within the envelope the flanges are thermally and electrically connected to alternate anode segments 10 as illustrated.

The anode segments 11, on the other hand, are electrically connected to each other by a radially extending annular flange 17, provided with a terminal lead portion 18 outside the envelope for connection to the high potential inner conductor of an associated coaxial transmission line. Each anode segment 11 is preferably connected at about its midpoint to the flange so as to provide on opposite sides of the flange oppositely extending longitudinal portions 19 and 21, the portions 19 extending toward the collector and the portions 21 extending toward the opposite end of the envelope. In this way each of the high potential anode segments 11 is only onehalf the over-all length of the anode, thus reducing the length of the lever arm subject to vibration and providing a more rugged structure. As shown, the flange 17 lies between the ground plane flanges 14, and the free ends of the longitudinal anode portions 19' and 21 are spaced from the associated ground plane flanges 14.

Since the inner conductor of the associated transmission line connected to the anode segments 11 must be at a high radio-frequency potential, these segments are at a high potential with respect to the anode segments 10. It is therefore necessary to electrically insulate the annular ground plane flanges 14 from the annular flange 17. For this purpose the envelope is preferably fabricated from a multiplicity of axially aligned tubular dielectric portions 22, 24, 26, 27 and 28, preferably ceramic, alternately stacked with and brazed to the associated metallic members. As shown in FIGURE 1, the ceramic portion 22 is hermetically interposed between the collector and the nearest flange 14 connecting segments 10 with terminal lead 16 outside the envelope. This ceramic is preferably fabricated from beryllia (BeO) in order to provide an electrically insulated but thermally conductive path permitting conduction of heat from the ground plane flange 14 to the collector. Ceramic envelope portions 24 and 26 are interposed on opposite sides of the flange 17 and cooperate with ceramic portion 22 to completely electrically isolate the high-frequency anode segments from the collector and insulate the ground potential anode segments 10 from the high potential segments. Both envelope portions 24 and 26 are preferably beryllia in order to provide high thermal conductance from the anode segments outward to the collector. The tubular ceramic portion 27 is hermetically brazed to the flange 14 on the side thereof opposite tubular ceramic portion 26, and serves to electrically insulate control electrode 29 from the associated anode structure. The control electrode is supported on a radially extending flange 30 having a terminal portion 31 outside the envelope.

As illustrated in FIGURE 1, the control electrode is preferably tubular in form and'at one end is axially spaced a small amount from the array of anode segments. This end of the control electrode is provided with a tapered end portion 32 in the nature of an external chamfer. Ceramic envelope portion 28 is hermetically interposed between the flange 30 and a sealing flange 34, and the union to flange 34 is strengthen by a back-up ring 36 of ceramic.

The opposite end of the envelope from the collector 3 is closed by the metallic plate 4 which serves to support coaxially within the envelope a centrally disposed metallic tube 37 preferably fabricated from a material that is electrically conductive but substantially thermally nonconductive. The tube extends coaxially into the interior of the cold cathodeand snugly surrounds the guide post 9 in a sliding fit while remaining electrically and thermally insulated from the collector. The lower end of the tube 37 is flared as at 38 and spot-welded to a conical metallic shell 39 having on its base end 41 a flange 42 secured to end plate 4 as by brazing or spot-welding. The conical shell 39 is preferably fabricated from the same material as the tube 37 so as to be substantially thermally nonconductive.

As shown in FIGURE 1, the tube 37 serves to rigidly support an electron emissive hot cathode 43 coaxially within the control electrode 29. The emissive portion 44 of the cathode is formed on the outer cylindrical periphcry of a tubular machined base member 45 having a reduced diameter integral extension 46 spot-welded to tube 37 within the open end of the cold cathode. As shown in the drawing, both the cathode base member and the tube 37 are spaced from the cold cathode. The amount of space between the cold and hot cathode is significant in that it should be suflicient to provide electrical and thermal insulation between the cold cathode and the hot cathode support member, but not so great as to form a discontinuity in the electrostatic field in the annular space 47 between the control electrode and the cathode. It has been found that a spacing of about .010" gives satisfactory results. It will be apparent that the tube 37, because of its extreme thinness, in order of about .005", and because of the slidable nature of its connection with the ceramic post 9, functions as a very eflicient heat dam between the collector and the hot cathode, and permits unlimited expansion and contraction of the envelope without imposing stress on the tube. Such expansion and contraction is of course undesirable and should be avoided to the extent possible by selection of materials having compatible coeflicients of thermal expansion and contraction. Additionally, the ceramic post 9, preferably fabricated from alumina, also functions as a heat dam. While functioning to impede the flow of heat from the collector to the cathode, these elements also cooperate to rigidly support the cathode within the control electrode against movement caused by shock and vibration. Another advantage of this cooperative relationship is that this envelope portion may be fabricated to close specifications as a separate unit and then assembled with the other envelope portion which has also been fabricated as a separate unit.

In order to energize the hot cathode, the magnetron is provided with a heater 48 spirally wound about the centrally disposed tube 37 in position to radiate heat to the cathode base member 45 and therethrough to the electron emissive surface itself. One terminal end 49 of the heater is connected by an appropriate lead 50 with the inner radially extending edge portion 51 of an annular metallic sealing ring, the cylindrical outer flange 52 of which is heliarc-welded to the sealing flange 34 as shown. The other end 53 of the heater is spot-welded as at 54 to the central tube 37. An annular ceramic envelope member 56 is brazed to one side of the sealing ring 51, and a back-up ceramic 57 is brazed to the opposite surface of the sealing ring 51. When the tube is assembled, this back-up ceramic lies in slidable abutment with the back-up ceramic 36. An aperture 58 formed in the two back-up ceramics provides a pasasgeway for evacuation of the envelope through tubulation 59. The ceramic envelope member 56 is hermetically brazed also to a flange portion 61 formed on the end plate 4, and thus completes the envelope. A back-up ceramic 62 brazed to the flange 61 on the opposite side thereof from the ceramic envelope portion 56 strengthens the union between the ceramic and metal parts.

In use, the magnetron is mounted between the poles of a permanent magnet providing a magnetic field exactly aligned with the longitudinal axis of the envelope. It is especially important that the magnetic field be axially aligned with the anode segments and 11, and that the magnetic flux density and distribution be such as to substantially uniformly fill the annular interaction space 13 defined by the anode segments and the cold cathode. Thus, when a high potential is placed on the control electrode, and the cathode heater is energized from an appropriate source of power, electrons are emitted and accelerated radially outwardly from the cathode until the influence of the axially directed magnetic field imposes on the electrons a circumferentially directed velocity component. Because of the crossed field effects of the electrostatic and magnetic fields, the electrons will spiral about the cold cathode while moving axially toward the collector. As they pass through the interaction space 13, the electron stream will cause high radio-frequency fields to appear across the gaps between the anode segments 10 and 11. Some of the electrons will spiral back and strike the cold cathode. The impact of each electron, multiplied many times, tends to heat the cold cathode. So that this heat may be efliciently dissipated, it is important that the cold cathode be directly connected to the collector.

I claim:

1. A voltage-tunable magnetron comprising an evacuated envelope including a composite hollow intermediate envelope portion and first and second metallic end cap structures hermetically closing opposite ends of the intermediate envelope portion, said first metallic end cap structure including a collector electrode having a dielectric post fixed thereon and an elongated hollow metallic extension surrounding the dielectric post and extending into the hollow intermediate envelope portion to constitute a cold cathode therewithin, an anode within the envelope having circumferentially spaced longitudinally extending interdigital segments radially spaced from the cold cathode to define therewith an annular interaction space, a support member fixed to the second end cap structure adjacent one of its ends and adjacent its other end engaging the dielectric post, an indirectly heated cathode axially displaced from the anode and rigidly supported on the support member in thermally and electrically insulated relation with respect to the cold cathode, heater means to energize the cathode, and a tubular control electrode arranged concentrically about the cathode.

2. The combination according to claim 1, in which said elongated hollow metallic extension surrounding the dielectric post is integral with the collector and extends coaxially into the hollow intermediate envelope portion.

3. The combination according to claim 1, in which said dielectric post is secured to said collector by one of its ends and is disposed concentrically within said elongated hollow metallic extension, the end of the post remote from the collector lying intermediate the ends of said elongated hollow metallic extension.

4. The combination according to claim 1, in which said composite hollow intermediate envelope portion includes a multiplicity of alternately stacked annular ceramic and metallic members hermetically united and porportioned to provide axially spaced radially extending metal flanges within the envelope and terminal leads outside the envelope, opposite ends of alternate longitudinally extending interdigital anode segments being electrically and thermally connected to a pair of said axially spaced radially extending metal flanges, the remaining interdigital anode segments being electrically connected intermediate their ends to another of the metal flanges axially spaced intermediate said flanges constituting said pair.

5. The combination according to claim 1, in which said support member is centrally disposed on said second end cap structure and extends coaxially into the elongated hollow metallic extension into sliding engagement with the dielectric post.

6. The combination according to claim 1, in which said indirectly heated cathode includes a tubular base member radially spaced about a portion of said support member and an integral extension thereon integrally united to the support member.

7. The combination according to claim 1, in which said support member comprises a substantially thermally nonconductive tubular metallic portion concentrically arranged with respect to the cathode, control electrode and said elongated hollow metallic extension, and a conical hollow shell poriton having itsapex end united to said tubular metallic portion and its base end united to said second end cap structure.

8. A voltage-tunable magnetron comprising an evacuated envelope including a composite hollow intermediate envelope portion having a multiplicity of alternately stacked annular ceramic and metallic members hermetically united and proportioned to provide axially spaced radially extending metal flanges within the: envelope and terminal leads outside the envelope, first and second metallic end cap structures hermetically closing opposite ends of the intermediate envelope portion, said first metallic end cap structure including a collector electrode having a dielectric post centrally secured by one of its ends on the collector and an integral elongated hollow metallic extension extending coaxially into the hollow intermediate envelope portion to constitute a cold cathode therewithin, an anode within the envelope having circumferentially spaced longitudinally extending interdigital segments radially spaced from the cold cathode to define therewith an annular interaction space, opposite ends of alternate longitudinally extending interdigital anode segments being electrically and thermally connected to a pair of said axially spaced radially extending metal flanges, the remaining interdigital anode segments being electrically connected intermediate their ends to another of the metal flanges axially spaced intermediate said flanges constituting said pair of flanges, a support member centrally disposed on said second end cap structure and extending coaxially into the elongated hollow metallic extension into sliding engagement with the dielectric post, an indirectly heated cathode axially displaced from said anode and having a tubular base member radially spaced about a portion of said support member and an integral extension thereon rigidly united to the support member in spaced relation to the cold cathode to support the cathode in thermally and electrically insulated relation with respect to the cold cathode, heater means to energize the cathode disposed between said tubular base member and the support member, and a tubular control electrode arranged concentrically about the cathode and supported on said hollow intermediate envelope portion.

9. In a voltage-tunable magnetron, a composite hollow envelope portion including a plurality of alternately stackedannular ceramic and metallic members hermetically united and proportioned to provide a plurality of axially spaced radially extending metal flanges within the envelope portion and terminal leads outside the envelope portion, an anode within the envelope portion having circumferentially spaced longitudinally extending interdigital segments arranged in a circular array, opposite ends of alternate longitudinally extending interidgital anode segments being electrically and thermally connected to two of the said axially spaced radially extending flanges constituting a pair, the remaining interdigital anode segments being electrically connected intermediate their ends to another of the metal flanges axially spaced intermediate said flanges constituting the pair, a tubular control electrode disposed within said composite hollow envelope portion and having its inner peripheral surface in axial alignment with the inner peripheral surfaces of said anode segments, one end of said tubular control electrode being axially spaced from said anode, and an end cap structure hermetically closing the end of the composite hollow envelope portion remote from the control electrode and including a collector electrode having an elongated hollow metallic extension thereon extending into the hollow envelope portion to constitute a cold cathode concentrically disposed in radially spaced relation within the circular array of interdigital anode segments and substantially coextensive therewith.

10. In a voltage-tunable magnetron, a first composite hollow envelope portion including a plurality of alternately stacked annular ceramic and metallic members hermetically united and proportioned to provide a plurality of axially spaced radially extending metal flanges within the first envelope portion and terminal leads outside the first envelope portion, an anode within the first envelope portion having circumferentially spaced longitudinally extending interdigital segments arranged in a circular array, opposite ends of alternate longitudinally extending interdigital anode segments being electrically and thermally connected to two of said axially spaced radially extending flanges constituting a pair, the remaining interdigital anode segments being electrically connected intermediate their ends to another of the metal flanges axially spaced intermediate said flanges constituting the pair, a tubular control electrode disposed within said first composite hollow envelope portion and having its inner peripheral surface in axial alignment with the inner peripheral surfaces of said anode segments, one end of said tubular control electrode being axially spaced from said anode, an end cap structure hermetically closing the end of the composite hollow envelope portion remote from the control electrode and including a collector electrode having an elongated hollow metallic extension thereon extending into the first hollow envelope portion to constitute a cold cathode concentrically disposed in radially spaced relation within the circular array of interdigital anode segments and substantially coextensive therewith, a second composite hollow envelope portion including a [plurality of alternately stacked annular ceramic and metallic members hermetically united, one of said metallic members constituting a metallic end cap structure hermetically closing one end of the second composite hollow envelope portion and another of said metallic members constituting a first sealing ring having a cylindrical flange thereon, a second sealing ring hermetically united to the adjacent end of said first composite hollow envelope portion and having a cylindrical flange thereon hermetically united to the cylindrical flange on said first sealing ring to hermetically seal the assembly of said first and second composite hollow envelope portions together, and a tubulation mounted on said second sealing ring and communicating with the interior of said composite envelope portions to enable evacuation of said assembly.

11. A voltage-tunable magnetron comprising a first composite hollow envelope portion including a plurality of alternately stacked annular ceramic and metallic members hermetically united and proportioned to provide a plurality of axially spaced radially extending metal flanges within the first envelope portion and terminal leads outside the first envelope portion, an anode within the first envelope portion having circumferentially spaced longitudinally extending interdigital segments arranged in a circular array, opposite ends of alternate longitudinally extending interdigital anode segments being electrically and thermally connected to two of said axially spaced radially extending flanges constituting a pair, the remaining interdigital anode segments being electrically connected intermediate their ends to another of the metal flanges axially spaced intermediate said flanges constituting the pair, a tubular control electrode disposed within said first composite hollow envelope portion and having its inner peripheral surface in axial alignment with the inner peripheral surfaces of said anode segments, one end of said tubular control electrode being axially spaced from said anode, an end cap structure hermetically closing the end of the composite hollow envelope portion remote from the control electrode and including a collector electrode having an elongated hollow metallic extension thereon extending into the first hollow envelope portion to constitute a cold cathode concentrically disposed in radially spaced relation within the circular array of interdigital anode segments and substantially coextensive therewith, a second composite hollow envelope portion including a plurality of alternately stacked annular ceramic and metallic members hermetically united, one of said metallic members constituting a metallic end cap structure hermetically closing one end of the second composite hollow envelope portion and another of said metallic members constituting a first sealing ring having a cylindrical flange thereon, a second sealing ring hermetically united to the adjacent end of said first composite hollow envelope portion and having a cylindrical flange thereon hermetically united to the cylindrical flange on said first sealing ring to hermetically seal the assembly of said first and second composite hollow envelope portions together, a tubulation mounted on said second sealing ring and communicating with the interior of said composite envelope portions to enable evacuation of said assembly, a substantially thermally nonconductive support member fixed on the metallic end cap structure associated with said second composite envelope portion and extending coaxially through said .control electrode into said cold cathode, dielectric means fixed on the collector and slidably engaging said support member within the cold cathode, and an energizable indirectly heated tubular cathode supported on said support member, the emissive surface of said cathode being axially displaced from said anode and cold cathode and axially aligned with said cold cathode.

References Cited by the Examiner UNITED STATES PATENTS 2,463,416 3/1949 Nordsieck 3l539.73 X 3,020,445 2/1962 Weinstein 3l539.63 3,084,280 4/1963 McLaughlin 3l5-39.73

HERMAN KARL SAALBACH, Primary Examiner. ARTHUR GAUSS, Examiner.

S. CHATMON, JR., Assistant Examiner. 

1. A VOLTAGE-TUNABLE MAGNETRON COMPRISING AN EVACUATED ENVELOPE INCLUDING A COMPOSITE HOLLOW INTERMEDIATE ENVELOPE PORTION AND FIRST AND SECOND METALLIC END CAP STRUCTURES HERMETICALLY CLOSING OPPOSITE ENDS OF THE INTERMEDIATE ENVELOPE PORTION, SAID FIRST METALLIC END CAP STRUCTURE INCLUDING A COLLECTOR ELECTRODE HAVING A DIELECTRIC POST FIXED THEREON AND AN ELONGATED HOLLOW METALLIC EXTENSION SURROUNDING THE DIELECTRIC POST AND EXTENDING INTO THE HOLLOW INTERMEDIATE ENVELOPE PORTION TO CONSTITUTE A COLD CATHODE THEREWITHIN, AN ANODE WITHIN THE ENVELOPE HAVING CIRCUMFERENTIALLY SPACED LONGITUDINALLY EXTENDING INTERDIGITAL SEGMENTS RADIALLY SPACED FROM THE COLD CATHODE TO DEFINE THEREWITH AN ANNULAR INTERACTION SPACE, A SUPPORT MEMBER FIXED TO THE SECOND END CAP STRUCTURE ADJACENT ONE OF ITS ENDS AND ADJACENT ITS OTHER END ENGAGING THE DIELECTRIC POST, AN INDIRECTLY HEATED CATHODE AXIALLY DISPLACED FROM THE ANODE AND RIGIDLY SUPPORTED ON THE SUPPORT MEMBER IN THERMALLY AND ELECTRICALLY INSULATED RELATION WITH RESPECT TO THE COLD CATHODE, HEATER MEANS TO ENERGIZE THE CATHODE, AND A TUBULAR CONTROL ELECTRODE ARRANGED CONCENTRICALLY ABOUT THE CATHODE. 